I scanned this in from Guidance Note 1
EARTHING OF FLUSH METAL ACCESSORY BOXES
It has been questioned whether flush metal accessory boxes come within the definition of 'exposedconductive-parts', which require to be connected to the circuit protective conductor.
An exposed-conductive-part is defined as follows:
'A conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions'.
In normal use, such accessory boxes cannot be touched. Only when an accessory is removed can its metal accessory box be touched.
However, the view of the Wiring Regulations National Committee is that flush metal accessory boxes should be considered to be exposed-conductive-parts and connected to the main earthing terminal by means of a circuit protective conductor.
Where an accessory does not have an earthing terminal incorporated, such as certain light switches, the circuit protective conductor should be terminated at the earthing terminal in the accessory box.
Where an accessory has an earthing terminal incorporated, the protective conductor should be connected at the earthing terminal. Accessories which have an earthing terminal incorporated, such as socket-outlets, normally have an earth strap connecting the earthing terminal to one or both of the fixing holes. Flush metal accessory boxes usually have at least one fixed lug. The Wiring Regulations National Committee has agreed that such boxes may be earthed via the tight metal-to-metal contact of the fixing screw in the fixed lug. Consequently, unless both lugs are of the adjustable type, it is not necessary to connect the earthing terminal of the accessory to the earthing terminal in the associated flush metal accessory box by a separate protective conductor (i.e. an earthing tail).
Where the circuit protective conductor is formed by conduit, trunking, ducting or the metal sheath and/or armour of a cable, an earthing tail must be fitted.
Where there is any doubt about the continued effectiveness, reliability and permanence of an earthing connection formed by the screws between an accessory and a metal box, it is recommended that a suitably sized earthing tail is provided.